1. Field of the Invention
The present invention relates to a radio communication apparatus and, more particularly, to an apparatus where the level of its transmission output is variable.
2. Description of the Related Art
In a duplex mobile telephone system portable telephone or car telephone cellular phone system, a base station is capable of controlling or designating the transmission output level of a mobile station. The reason is based on the purpose of suppressing the transmission output of a mobile station to the minimum value required for the base station, so as to increase the number of mobile stations, which can utilize the system, by using the same channel even in a cell (service area) of an other base station.
In the TACS (Total Access Communication System), the control or designation of the transmission output of a mobile station is realized by transmission of a 3-bit code termed MAC (Mobile Attenuation Code) from the base station to the mobile station. An exemplary construction for controlling the level of a transmission output in accordance with the MAC will now be described below with reference to FIG. 1.
FIG. 1 shows a portable telephone of analog cellular system including a mouthpiece 11, an audio circuit 12, a transmitting circuit 13, a power amplifier 14, a drive stage (former stage) 141 of the amplifier, an output stage (last stage) 142 of the amplifier, a duplexer 15, a transmitting/receiving antenna 16, a receiving circuit 17, and an earpiece 18. In the amplifier 14, the drive stage 141 performs a class AB operation, while the output stage 142 performs a class C operation.
When a sound signal from the mouthpiece 11 is supplied via the audio circuit 12 to the transmitting circuit 13, it is converted into an up-channel transmission signal (FM signal) S13. This transmission signal S13 is supplied to the amplifier 14 and amplified. Then this transmission signal thus amplified S13 is supplied via the duplexer 15 to the antenna 16, from which the signal S13 is transmitted to the base station.
Meanwhile a down-channel transmission signal from the base station is received at the antenna 16. The signal thus received is supplied via the duplexer 15 to the receiving circuit 17, so that the sound signal from the talker is extracted. Thereafter the sound signal thus extracted is supplied via the audio circuit 12 to the earpiece 18.
Denoted by reference numeral 21 is a system controller which consists of a microcomputer. The system controller, transmitting circuit 13 and the receiving circuit 17 are connected to each other via a modem circuit 22, whereby a predetermined command signal or data such as a MAC can be communicated between the system controller 21 and the base station. Further the audio circuit 12, the transmitting circuit 13 and the receiving circuit 17 are controlled by the system controller 21 in response to the command signal or the data received from the base station. Denoted by 23 is an operation key such as a dial key or a talk key.
Reference numeral 30 denotes an APC (Auto Power Control) circuit for controlling the magnitude of the transmission output in accordance with the MAC. More specifically, AOC data D31 is obtained from an output port of the system controller 21 having a latch function. The AOC data D31 is changed in accordance with the MAC transmitted from the base station, so as to control the magnitude of the transmission output.
The data D31 is supplied to a D-A converter 31, where a digital-to-analog conversion is executed to convert the data D31 into an analog voltage V31. The voltage V31 thus obtained through the digital-to-analog conversion is supplied as a reference voltage to a voltage comparator 32. Further the transmission signal S13 from the power amplifier 14 is supplied to a detector 33, which then outputs a DC voltage V33 of a level corresponding to the level or magnitude of the transmission signal S13, and then the voltage V33 is supplied to the comparator 32.
The emitter-collector junction of a transistor 35 is connected in series between a battery 34 for a power supply and a power line of the drive stage 141, and a comparison output voltage V32 obtained from the comparator 32 is supplied to the base of the transistor 35 as a control signal for the transistor 35. To the output stage 142, the voltage of the battery 34 is supplied directly.
When the comparison output voltage V32 is changed in the portable telephone of the construction described above, the operating voltage supplied to the drive stage 141 is also changed in accordance with such change of the voltage V32, so that the level of the transmission signal S13 supplied from the drive stage 141 to the output stage 142 is changed in accordance with the voltage V32. Consequently, the level of the transmission signal S13 produced from the output stage 142 is changed correspondingly to the voltage V32.
Since feedback is being executed at this time by the comparator 32 in such a manner as to render V33 =V31, the level (=V33) of the transmission signal S13 produced from the output stage 142 is equalized to the level of the voltage V31.
Accordingly, it is possible to control the level of the transmission signal S13 obtained from the output stage 142 by controlling the value of the data D31 in the system controller 21, whereby the base station is able to control the magnitude of the transmission output from the portable telephone in accordance with the MAC.
In the portable telephone system, as described above, the level of the transmission output of a mobile station is suppressed to its minimum necessary value.
In a power amplifier, it is generally customary that, when the level of an input signal is high and an output is maximum, the collector voltage of an output transistor is fully swung substantially from the ground potential to the operating voltage which is equal to the supply voltage, whereby the efficiency of the amplifier is maximum. However, when the input signal level is low and the output magnitude is small, such full swing of the collector voltage is not performed, so that the efficiency of the amplifier is lowered.
Therefore, as graphically shown in FIG. 2, the efficiency n of the output stage 142 in the power amplifier 14 becomes maximum when its output PWR is in the proximity of the maximum thereof. Consequently, if the transmission output is reduced by lowering the drive level of the transmission signal S13 with respect to the output stage 142 as mentioned, there arises a problem that the efficiency at the time of a small transmission output is greatly lowered in comparison with the efficiency at the time of the maximum output.
Meanwhile, when the magnitude of the transmission output is small, a great voltage drop is caused in the transistor 35 to consequently lower the operating voltage supplied to the drive stage 141, whereby the loss in the transistor 35 is increased.
Accordingly, even if the portable telephone is used in any place under satisfactory conditions where its required transmission output is small, there exists a problem that its power consumption is not reduced in comparison with the decrease of the transmission output. Such a problem brings about a disadvantage that the service life of the battery 34 is shortened to eventually shorten the effective usable time of the portable telephone.